Travelling wave tubes



July 16, 1963 c. R. BUCKLEY ETAL 3,

TRAVELLING WAVE TUBES Filed Dec. 8, 1960 INVENTQRS ATTQRNEY S United States Patent ,0

3,098,169 TRAVELLING WAVE TUBES Charles Raymond Buckley and John Mervyn Tompsett, Chelmsford, England, assignors to English Electric Valve Company Limited, London, England, a British company Filed Dec. 8, 1960, Ser. No. 74,521 Claims priority, application Great Britain Feb. 8, 1960 7 Claims. (Cl. 315--3.5)

This invention relates to travelling wave tubes and, more particularly, to travelling wave tubes of the kind in which the delay line thereof, which is commonly a wire helix, is mounted accurately co-axially with the tube envelope by means of a number of insulating support rods which extend beyond the delay line at least at one end usually at both. In such tubes as normally constructed at present the ends of the delay line are continued into wires lying parallel to the axis, and which serve the purpose of coupling to input and output transmission lines or waveguides. Each of these wires is connected to a suitable electrical termination such as a choke which is also usually supported by the insulating support rods beyond the ends of the delay line.

Known travelling wave tubes of the kind referred to above have been found to be unstable in operation. For example, the beam current reaching the delay line, which in normal operation is small, is sometimes subjected to large variations. It is the object of this invention to provide travelling wave tubes which are free of the above mentioned instability.

The cause of this instability is rather obscure but it is at present believed to be due to the charging of the support rods beyond the ends of the delay line due either to bombardment by primary electrons from the beam or by secondary electrons emitted from parts of the structure of the tube. The accumulated charge is believed to distort the beam thus causing it to strike the delay line and so aggravate the condition. Whether or not this explanation is correct, travelling wave tubes in accordance with the present invention are, in fact, free of the above mentioned instability.

According to this invention the delay line support rods of a travelling wave tube, of the kind in which the delay line is supported co-axially with the tube envelope by a number of insulating rods which extend beyond said line at least at one end thereof, are provided at least over the extending end or ends thereof with a coating of conductive material connected to one of the tube electrodes.

Preferably said coating is made of high surface resistance, for example by being formed with one or more small gaps therein.

In a preferred embodiment the coating on each support rod is in the form of a helical track around it and in contact at one end with a choke which is carried on the rods, the coating extending at the other end to a point close to but spaced from the delay line where it touches said rod.

The invention will now be further described with reference to the accompanying drawing which is a diagrammatic illustration of a preferred embodiment of the invention.

Referring to the drawing, 1 is the envelope of a travelling wave tube having an electron gun 2 which is arranged, in known manner, to project an electron beam along the tube axis, the beam being finally collected by the collector electrode 10. A delay line in the form of a wire helix 3 is accurately co-axia lly mounted in the tube envelope and is held in position by means of insulating support rods. For simplicity in drawing only two such rods are illustrated and designated with the numeral 4 3,098,169 Patented July 16, 1963 although, in practice, there will normally be three rods symmetrically disposed around the tube envelope axis. The ends of the helix continue into straight wires 5 and 6 parallel to the axis and which terminate in choke electrodes 7 and 8 respectively, each choke being of cylindrical form and supported on the rods 4. In operation the tube is mounted with the wires 5 and 6 aligned with input and output waveguides or transmission lines respectively. As so far described the tube is as well known.

-In accordance with the present invention the portions of the support rods 4 extending between the ends of the delay line and the chokes are provided with helical tracks 9 of colloidal graphite. Each of these helical tracks is in contact with one or the other of the choke electrodes at one endas is seen in the drawing each track extends for some way along the rod within the chokeand at the other end stops just short of the helix 3.

In operation any electrons which bombard the support rods outwardly of the ends of the delay line will be conducted away along the helical track 9 to the choke and hence to the delay line and so will not build up a large charge on the insulating rods. Furthermore, due to the fact that each of the coatings of colloidal graphite is of long effective length due to its helical form, it presents a high impedance to energy radiated to or from the straight wire 5 or 6 and therefore does not adversely affect the transfer of energy to or from the delay line.

The coating may be made by placing a mask consisting of two colinear cylindrical tubes with a helical wire between them over the end of the support rod to be coated. The rod, together with the mask, is rotated in a chuck and sprayed with a suspension of colloidal graphite in a mixture of water and potassium silicate, and the sprayed rods are then baked at a high temperature to dry out the coating and to adjust the resistance of the coating to a desired value. The resistance obtained is dependent on the composition of the suspension, the duration of the spraying time and the duration of the final baking time.

In one experimentally tested embodiment of the invention each of the helical graphite tracks had a width of about 0.050" and 12 turns per inch to give a value of resistance between the ends of the graphite helix of 100,000

ohms. The embodiment was found very satisfactory on test.

We claim:

1. A travelling wave tube having a tube envelope, insulating support rods, a delay line and electrodes for connecting the delay line into circuit, the delay line being supported coaxially within the tube envelope by said insulating support rods, said rods extending beyond said delay line at one end thereof and wherein each of said insulating support rods is provided over the extending end thereof with a coating of conductive material connected to one of the said electrodes to prevent electrical charging phenomena on the said end of. the rod by providing a conductive discharge path for electrical charges on the said end of the rod.

2. A travelling wave tube according to claim 1, wherein said one electrode is a choke electrode.

3. A tube as claimed in claim 1 and wherein said one electrode is connected to one end of the delay line and is situated outwardly of said delay line and is supported by the insulating support rods and wherein the rod coatings at the said end are connected to said one electrode and extend therefrom to a point adjacent but spaced and insu- 'lated from said delay line end.

4. A tube as claimed in claim 1 wherein the coatings are of deposited graphite.

5. A tube as claimed in claim I]. wherein the coatings are of high surface resistance.

6. A tube as claimed in claim 1 wherein the coatings nomena on said insulated rods by providing a conductive are formed each with a narrow helical gap therein. discharge path for electrical charges on said rods.

7. A travelling wave tube having electrodes insulating support rods and a delay line which is supported coaxially References Cited in the file of this patent within the tube envelope by said insulating support rods, 5 said rods extending beyond said line at least at one end UNITED STATES PATENTS thereof and wherein said insulating support rods are pro- 3,005,126 Cutler -1 196'1 vided at least over the extending end thereof with a coat- 3,026,445 Staerck M2113 1962 ing of conductive material, which coating is connected to one of the tube electrodes whereby said coating and said 10 FOREIGN PATENTS one electrode act to prevent electrical charging phe- 1,064,126 Germany Aug.27, 1959 

1. A TRAVELLING WAVE TUBE HAVING A TUBE ENVELOPE, INSULATING SUPPORT RODS, A DELAY LINE AND ELECTRODES FOR CONNECTING THE DELAY LINE INTO CIRCUIT, THE DELAY LINE BEING SUPPORTED COAXIALLY WITHIN THE TUBE ENVELOPE BY SAID INSULATING SUPPORT RODS, SAID RODS EXTENDING BEYOND SAID DELAY LINE AT ONE END THEREOF AND WHEREIN EACH OF SAID INSULATING SUPPORT RODS IS PROVIDED OVER THE EXTENDING END 